1. Field of the Invention
The present invention relates generally to offshore working platforms which are self-propelled or towed to a shallow water well site and can be jacked-up above the water surface for use in drilling and production activities, and more particularly to cranes used on such platforms.
2. Description of the Prior Art
Typically, self-elevating lift barges or other self-elevating or jack-up platforms are used in offshore shallow water drilling and production activities. These self-elevating platforms or rigs typically incorporate a deck-mounted crane. These cranes are essential to the operation of such vessels or platforms and often become more important to the end user of these boats or platforms than the boat or platform itself.
The cranes typically used on jack-up boats or platforms in the past have included hydraulic cranes employing a box boom and using hydraulic cylinders to lift the boom. These cranes are very popular due to their short swing radius. However, in the recent past jack-up boat and platform sizes have increased substantially resulting in an increased demand being placed on a deck mounted crane which frequently exceeds the lifting capabilities of a typical cylinder lift crane. Since cylinder lift cranes rely on the strength of the boom to suspend and support the load, the heavier the load, the stronger or heavier the crane boom must be to support it. The heavier crane booms create several problems for a jack-up boat or platform, the most significant of which is listing while the boat or platform is underway.
In order to economize on weight, the alternative to a cylinder lift crane has been a conventional lattice-type boom which is tip supported. Although the boom on tip supported cranes is much lighter, all lattice boom cranes suffer from a major drawback of a very large overhanging gantry which increases the swing radius thereby taking up valuable deck space.
In contrast to cylinder lift cranes, which can raise their boom to almost a vertical position thereby enabling a load to be placed only a few feet from the crane base on the deck, conventional lattice boom cranes are not capable of raising the boom to a near vertical position. Accordingly, a tip-supported lattice boom crane which is mounted on deck suffers from an additional drawback of not being able to turn a full 360.degree. due to interference from the long, extended legs mounted on the periphery of the platform or deck of the jack-up boat.